Common austenitic stainless steels contain a maximum by weight percent of 0.15% carbon, a minimum of 16% chromium and sufficient nickel and/or manganese to retain a face centered-cubic (FCC) austenitic crystal structure at cryogenic temperatures through the melting point of the alloy. Austenitic stainless steels are nonmagnetic non-heat-treatable steels that are usually annealed and cold worked. Common austenitic stainless steels are widely used in power generating applications; however, they are becoming increasingly less desirable as the industry moves toward higher thermal efficiencies. Higher operating temperatures in power generation result in reduced emissions and increased efficiencies. Conventional austenitic stainless steels currently offer good creep strength and environmental resistance up to 600-700° C. However, in order to meet emission and efficiency goals of the next generation of power plants structural alloys will be needed to increase operating temperatures by 50-100° C.
Austenitic stainless steels for high temperature use rely on Cr2O3 scales for oxidation protection. These scales grow relatively quickly, and do not function well in environments containing species like water. High nickel austenitic stainless steels and nickel based superalloys can meet the required property targets, but their costs for construction of power plants are prohibitive due to the high cost of nickel. Creep failure of common austenitic stainless steels such as types 316, 321, and 347 has limited the use of these.
A new class of austenitic stainless steels has been recently developed to be more oxidation resistant at higher temperature—these are the alumina-forming austenitic (AFA) stainless steels. These alloys are described in Yamamoto et al. U.S. Pat. No. 7,754,305, Brady et al U.S. Pat. No. 7,744,813, and Brady et al U.S. Pat. No. 7,754,144, the disclosures of which are hereby incorporated fully by reference.